1. Field of the Invention
The present invention relates to a process for the mass production, in two distinct phases, of vinyl chloride based polymers and copolymers having a low content of the coarser particulates, as well as to the particular polymers and copolymers produced thereby.
2. Description of the Prior Art:
The mass or bulk polymerization of a monomer formulation based on vinyl chloride initially progresses in the liquid monomer phase. Because of the insolubility of the polymer or copolymer in the monomer composition, the reaction product separates in the solid state during the course of polymerization. When the content of the reaction medium in polymer or copolymer attains a value on the order of 15% by weight, its consistency becomes comparable to that of agitated, curdled milk. This consistency prevails until the monomer composition no longer defines a continuous liquid phase; this occurs when the amount by weight of the reaction medium in polymer or copolymer attains a value on the order of 25%. Beyond this value, the reaction medium approximates a powdery state.
It has already been proposed to this art to prepare polymers and copolymers based on vinyl chloride by the polymerization or copolymerization, in mass, in two phases, by a process consisting of conducting the polymerization or copolymerization [1] by maintaining the highest possible velocity of agitation during an initial phase, until a rate of conversion of the monomer formulation on the order of 7 to 15% and preferably on the order of 8 to 12%, is attained, [2] then reducing the velocity of agitation during a second phase in the reaction to the lowest possible value, but still sufficient to assure good heat exchange within the polymerization recipe, and [3] maintaining this rate to the end of said reaction. According to these techniques, the two phases, the first performed under a high turbulence agitation, the second with a low rate of agitation, may be performed either in a single autoclave or polymerization vessel or in different autoclaves, the phases being designated prepolymerization and polymerization, respectively, and the appropriate apparatus therefore being designated prepolymerizer and polymerizer, also respectively.
Also in accordance with the aforesaid techniques, the second phase is conducted either with a reaction medium consisting solely of the monomer/polymer composition originating from the first phase of polymerization, or together with said monomer/polymer composition, a supplementary vinyl chloride base monomer composition, identical to or different from the composition employed during the first phase, and one or more polymerization initiators.
These particular processes and their noted variants are described in detail in the French patents and respective certificates of addition (CA) thereto which follow (all being assigned to Produits Chimiques Pechiney Saint-Gobain): French Pat. No. 1,357,736, CA's 83,377, 83,383 and 83,714; French Pat. No. 1,382,072, CA's 84,958, 84,965, 84,966 [all corresponding to U.S. Pat. No. 3,522,227], 85,672 [corresponding to U.S. Pat. Nos. 3,562,237 and 3,544,280] and 89,025 [corresponding to U.S. Pat. No. 3,558,584]; French Pat. No. 1,436,744, CA's 87,620, 87,623, 87,625 and 87,626 [all corresponding to U.S. Pat. No. 3,687,919]; French Pat. No. 1,450,464 [corresponding to U.S. Pat. No. 3,687,923]; French Pat. No. 1,574,734 [corresponding to U.S. Pat. No. 3,683,051] and French Pat. No. 1,605,157 [corresponding to British Specification No. 1,260,377] [all hereby expressly incorporated by reference].
Such processes and variations thereon reproducibly yield vinyl chloride polymers and copolymers, which, after screening, exhibit the important properties of high apparent density and narrow granulometric distribution, which may be controlled as desired.
However, prior to screening, these polymers and copolymers prepared according to the foregoing processes have a high content of coarse products having practically no commercial value. Thus, that fraction thereof passing through a 250 micron mesh screen, variable as a function of the AFNOR viscosity index of the polymer or copolymer, determined by the NFT 51013 standard, does not exceed 94% by weight.